I'm not super familiar with the issue, I've seen this discussed on this mailing list, and somebody else must have a better answer, but anyway . . .
Hi group,
I am reading in a file created from one of the CMIP6 models. It appears to have a coordinate transformation embedded as metadata, which one might think would yield different z-levels for the 4D variable (vo) at each horizontal location. However, when I read the file into ferret v7.5, I get vo appearing on a single z-axis (which ferret labels as "lev"), ranging from 1 to 7250. That axis has the same values as the "zlev" variable in the original file. Now, "zlev" is in fact used in the depth formula, but it is *not* the only term - other terms in the equation suggest that z-levels should vary by location and time (see the formula listed in the ncdump below). Is ferret in fact regridding to a uniform z-axis based on zlev, or is it erroneously assuming all is uniform?
Ferret just shows information contained in your netCDF file. It doesn't automatically regrid the data for you.
Your 4D variable seems to be defined on the sigma coordinates for levels k = 1, 2, 3, . . . , nsigma
and on the level coordinates for the deeper levels (k = nsigma + 1, . . . ).
for k <= nsigma: